October 20, 2012

The Economist looks at the performance of electric cars, fuel-cell cars, and nitrogen-powered cars:

As long as its storage container is well insulated, liquid air can be kept at atmospheric pressure for long periods. But on exposure to room temperature, it will instantly boil and revert back to its gaseous state. In the process, it expands 700-fold — providing the wherewithal to operate a piston engine or a turbine.

Liquid nitrogen does an even better job. Being considerably denser than liquid air, it can store more energy per unit volume, allowing cars to travel further on a tankful of the stuff. Weight for weight, liquid nitrogen packs much the same energy as the lithium-ion batteries used in laptops, mobile phones and electric cars. In terms of performance and range, then, a nitrogen vehicle is similar to an electric vehicle rather than a conventional one.

The big difference is that a liquid-nitrogen car is likely to be considerably cheaper to build than an electric vehicle. For one thing, its engine does not have to cope with high temperatures — and could therefore be fabricated out of cheap alloys or even plastics.

For another, because it needs no bulky traction batteries, it would be lighter and cheaper still than an electric vehicle. At present, lithium-ion battery packs for electric vehicles cost between $500 and $600 a kilowatt-hour. The Nissan Leaf has 24 kilowatt-hours of capacity. At around $13,200, the batteries account for more than a third of the car’s $35,200 basic price. A nitrogen car with comparable range and performance could therefore sell for little more than half the price of an electric car.

A third advantage is that liquid nitrogen is a by-product of the industrial process for making liquid oxygen. Because there is four times as much nitrogen as oxygen in air, there is inevitably a glut of the stuff — so much so, liquid nitrogen sells in America for a tenth of the price of milk.

1 Comment

Sounds good until theory meets up with reality. Even if one assumes none of the losses typical in a pneumatic system (about 30%), and ignores the substantial cryogenic tank volume penalty (nearly 100%, about double the physical size) due to the required insulation, the numbers still don’t pan out.

In an average vehicle with a 20 gallon (76 liter) liquid nitrogen tank, using the 700 to 1 expansion factor noted in the article, you get 53,000 liters of gaseous nitrogen. With a 5-liter pneumatic engine, that gives you 10,600 revolutions, which at 800 RPM (slow for a car engine), empties the tank in a little over 13 minutes, unless I’ve missed something.

Then there’s the cost of liquid nitrogen. Right now it’s basically a waste product (hence the low price) in the liquid oxygen production process because the market for liquid oxygen is astronomically larger than the market for liquid nitrogen. The volume of nitrogen required to power a commercially viable nitrogen vehicle market would reverse the waste product scenario and current price ratio between liquid oxygen (about $150.00/gallon) and liquid nitrogen (about $2.00/gallon). And ultimately, it takes a lot of electricity or some other primary energy source to compress the nitrogen to cryogenic pressures, so there’s no real benefit to the environment.